Angular rate of rotation about a given coordinate axis may be measured by moving (e.g., vibrating) an accelerometer along an axis normal to the accelerometer's sensitive axis and normal to the rate axis about which rotation is to be measured. For example, consider a set of X, Y, Z coordinate axes fixed in a body whose rotation rate is to be measured and an accelerometer also fixed in the body with its sensitive axis aligned along the Z axis. If the angular rotation vector of the body includes a component along the X axis, then periodic motion of the accelerometer along the Y axis will result in a periodic Coriolis acceleration acting in the Z direction that will be sensed by the accelerometer. The magnitude of the Coriolis acceleration is proportional to the rotation rate about the X axis. As a result, the output of the accelerometer includes a DC or slowly changing component that represents the linear acceleration of the body along the Z axis and a periodic component that represents the rotation of the body about the X axis. The accelerometer output can be processed, along with the outputs of accelerometers that have their sensitive axes in the X and Y directions and that are moved along the Z and X axes, respectively, to yield linear acceleration and angular rate about the X, Y and Z axes.
Commonly assigned U.S. patent application, Ser. No. 075,039, entitled, "A Frame Assembly and Dither Drive for a Coriolis Rate Sensor," filed on July 17, 1987, describes a preferred embodiment for a rotation rate sensor and a drive assembly for vibrating two accelerometers along an axis normal to their sensitive axes. The accelerometers are mounted in a frame so that their sensitive axes are parallel or antiparallel to each other. Two electromagnetic mounted on the drive assembly provide a magnetic driving force to vibrate or dither the frame and attached accelerometers back and forth, causing the frame to flex about a plurality of joints as the electromagnetics are alternately energized.
While some prior art dither drives have been operative to excite a rotation rate sensor to vibrate back and forth at its natural frequency, it is generally preferable to drive the sensor so that it vibrates at a connstant predefined frequency that may be different than the resonant frequency. Driving the Coriolis rate sensor at a known predefined frequency greatly facilitates processig the signals produced by the accelerometers and demodulation of the rotation rate signal.
The drive assembly described in the above-referenced patent application uses dual electromagnetic attractor coils that have several advantages over prior art solenoid and D-Arsonval torque coil dither drives. Regardless of the design of the dither drive, it is important that it be controlled to produce a stable dither motion. In addition, the control should provide a minimum settling time, i.e., the time required for the rate to achieve a stable dither amplitude should be insensitive to ambient temperature effects or voltage supply fluctuations, and should compensate for any phase shift between the driving force and the dither motion of the rate sensor that might create errors in the rotation rate signal due to harmonic distortion. Prior art dither drive controls have generally failed to address all these problems.
The electromagnetic attraction coils used in the above-referenced drive assembly present other control problems. The driving force of the electromagnetic coils is directly proportional to the dither drive current squared and inversely proportional to the varying dimension of the magnetic pole gap squared. Both of these nonlinear second order effects cause the dither motion of the rotation rate sensor to deviate from a pure sine wave and introduce errors in the rotation rate determination. Prior art dither drive controls are incapable of properly compensating for the nonlinearities of the electromagnetic attractor coil dither drive.
The present invention is directed to minimizing errors in the Coriolis rotation rate sensor output signal related to the dither drive, and compensating for problems that are common to all such drives, and for those specific to the electromagnetic attraction coil drive. Accordingly, it is an object of the present invention to provide a servo loop control for an attractor motor dither drive, which effects stable vibration of the Coriolis rate sensor and compensates for phase angle shift and nonlinearity in the driving force so that the dither motion of the rate sensor conforms to a sinusoidal function. This and other objects of the invention will be apparent from the description of the preferred embodiment that follows hereinbelow and the attached drawings.